The comparison of the water
balance of forested and treeless areas needs firstly reliable
estimates of precipitation. The differences in the amounts of
precipitation falling on and stored in the forest as compared to
the amounts in the adjacent, treeless areas (meadows, ploughland,
etc.) are usually accounted for by the differences in the
mechanism of formation, fall, and accumulation of precipitation
due to: (a) dynamic roughness of the underlying surfaces; (b)
condensation factors; (c) interception by vegetation; (d)
snow-pack formation factors. In the USSR many investigations have
been carried out to estimate the influence of each of the above
factors, separately and in combination, on the total quantity of
liquid and solid precipitation in the forest and in the open.

Rainfall

The forest creates additional
roughness for air masses moving in the lower atmosphere, slows
down their movement and causes turbulence, which leads to the
formation of ascending air fluxes, air cooling, cloud formation,
and, consequently, greater precipitation on forested areas. In
the USSR the idea was first proposed by A. M. Voeikov, at the end
of the nineteenth century, and later supported by M. A.
Velikanov, G. P. Kalinin, and D. L. Sokolovsky. This idea is
basic to many recent studies of forest influences on
precipitation. However, the effect may be explained by additional
roughness only for flat terrain; in hilly and mountainous areas
this factor can hardly be significant.

According to O. A. Drozdov, the
increase cannot reach more than 3 to 5% of the total annual
precipitation. However, substantially higher estimates have been
obtained using the data from several dozens of meteorological
stations and river basins and correlating either the measured
precipitation with the percentage of forest area around each
station within a radius of 25 to 30 km (or within 20 square
kilometres) or the average precipitation with the forest area
percentage of the river basin. The data were corrected for wind
effect. According to Kuznetsova's (1957) findings for the middle
Volga region, Voronezh and Tambov districts, each 10% increase of
forest area results in a 2% increase of precipitation on average.
For the regions of Moscow, Kuibyshev, Perm, and the Upper Volga
basin to Gorky, Rakhmanov (1962, 1971) obtained the rather large
increases of 8 to 10 mm in the annual totals for each 10%
increase of the forest area for the period 1945-1965. For the
south Siberian region Lebedev (1982) estimated that for each 10%
increase of forest area the increment of annual totals was equal
to 12-13 mm; Opritova (1978), using 40 drainage basins in
Primorie, found the increase of annual precipitation for each 10%
of added forest area to be about 25 mm. This increase, however,
may be due partly to more densely forested watersheds being
usually at higher elevations. Studies by Fedorov (1977) of forest
influence on the water balance for a number of years at the
Valdai branch of the State Hydrological Institute led him to
conclude that in the north-west of the European USSR annual
totals for forest are 35 to 44 mm (or 4-5% of the annual total)
greater than those above treeless areas due to the greater
dynamic roughness of the former; the precipitation increase is
confined mainly to the warm season and does not depend on the
type of forest. This was confirmed from studies of water balances
of river basins. For instance, Vodogretsky (1979) found that
annual precipitation above forests usually increases by 5 to 8%
in the forest and forest-steppe zones of the European USSR.

Thus, statistical estimates from
large numbers of stations in various regions of the USSR show
that the amount of precipitation falling on forests is somewhat
greater than that above treeless areas. However, this fact is far
from being universally recognized. As the increase is rather
small (4 to 6% according to the latest findings) and the accuracy
of precipitation measurements is rather low and varies for
precipitation gauges installed in the forest and in the field due
to different wind protection, many scientists in the USSR
(Bochkov 1970; Bulavko 1971; Krestovsky 1969a; Subbotin 1978) do
not accept these conclusions and do not take the phenomenon into
consideration when calculating water balances and developing
runoff forecasting methods. The problem needs further critical
investigation.

Horizontal Precipitation

The hydroclimatic role of forests
often includes condensation, dew, hoarfrost, etc., and horizontal
precipitation (mist catching). Reliable quantitative estimation
of this kind of precipitation is difficult and is not included in
standard observations of meteorological stations (save for
stating its occurrence). There are published, however,
quantitative assessments of horizontal precipitation, indicating
its importance in some regions. Such data are quite insufficient
to make reliable comparisons of the quantities of this kind of
precipitation in forests and in treeless areas.

According to Voronkov (1970),
based on four years' observations, condensation under the canopy
yields, on average, 10 to 15 mm during the warm period and 16 mm
during the cold period, or 25 to 30 mm per year (4% of the annual
precipitation). In cedar forests of the western Sayan Mountains
liquid horizontal precipitation penetrating the canopy was
estimated by Protopopov (1975) at about 7 to 10% of the total
precipitation. According to Lebedev's data (1982), in mixed
cedar-fir stands in the same region, liquid condensation makes up
15 to 30 mm, or about 2 to 3%, of the annual total; estimates
obtained for fields with a vegetative cover are about the same.
Condensation during the winter period is included in the
snow-storage estimates. In the virgin steppes of northern
Kazakhstan (Gidromet. 1966) maximum condensation is observed
during the summer-autumn season and reaches 30 to 40 mm (20-30%
of summer-autumn precipitation, or 9 to 11% of the annual total).

According to data summarized by
B. L. Sokolov (unpublished) condensation plays an important role
in the water balance of the permafrost regions of Central Yakutia
and Kolyma, reaching during the warm season 80 to 100 mm on the
average for a whole river basin. Condensation values may have a
wide range depending on the land surface; that is, from 50 mm to
300 mm, maximum values being observed on gravel and rocks without
a vegetative cover. Condensation leads to the increase of
specific river discharges in the permafrost zone equal to 0.5 to
2.0 litres km-2 s-1 and occasionally to 4
litres km-2 s-1 in small watersheds. Large
condensation values amounting to 100-150 mm during the warm
period are observed in the forests of Primorie. Still greater
condensation values are recorded in many other countries of the
world, in mountainous coastal regions in particular (see the
review by Rakhmanov 1981). Condensation in such regions amounts
to about 20 to 25%, and sometimes half, of the total annual
precipitation.

In the plains of the European
USSR, with a continental climate, condensation seldom exceeds 3
to 5% of the annual total and seems to be the same on average for
forests and treeless areas (Bulavko 1971; Gidromet. 1966). There
are two prerequisites for condensation: high air humidity and a
large daily air temperature variation. In forests air humidity is
higher, while large diurnal air temperature changes are typical
of the fields; hence these two factors are believed to compensate
each other and thus cause the similar condensation yields.

Maximum condensation amounting to
20 to 30 mm, 3 to 4% of the annual total, is observed at the
borders of forests, while minimum values, of 5 to 10 mm, or 1% of
the annual total, are recorded at distances of 100-150 m and
further into the forest. Winter condensation values are 10 mm and
5 mm (5% and 2% of winter precipitation) at the borders and deep
in the forest respectively. In fields annual condensation is
about 20 to 30 mm, or 3 to 4% of the anual total. Thus there is
hardly any reason to believe that condensation, or horizontal
precipitation, in the forest is greater than that in treeless
areas.

Interception Loss

For water balance studies one
also needs to carefully assess the portion of precipitation that
is intercepted by the canopy and lost by evaporation without
reaching the ground. Comparing observations from precipitation
gauges installed in the open and under the forest canopy,
Molchanov (1960) gives the following interception values: 34 to
46% of the annual total in spruce forest; 24 to 27% in pine
forest; 24% in birch; and 22% in oak stands. Similar interception
estimates were obtained by Voronkov (1970), Pobedinsky (1979),
and Idzon, Pimenova, and Tsyganova (1980) for the Moscow region
and by Bulavko (1971) for Byelorussia. In the Middle Urals and
the northern part of the European USSR, Bratsev and Bratsev
(1979) and E. P. Galenko et al. (Bratsev 1982) found that spruce
intercepts 29 to 46%, pine 25 to 30%, and deciduous species 15 to
24% of total precipitation either during the growing season or a
year. Interception values tend to be greater (40-60%) in dark fir
and spruce forest in the Carpathian and the Caucasus mountains
(Kaliuzhny, Pavlova, and Popov 1979; Rakhmanov 1981). For the
forests of the Asian part of the USSR, data by V. V. Protopopov,
L. K. Pozdnyakova, T. I. Tarakanova, A. P. Klintsov, and others
from different types of forests in Yakutia, the Sayan Mountains,
Primorie, and Sakhalin Island are summarized in V. V. Rakhmanov's
1981 review. The data show that interception values depend on
forest type and density and the precipitation type (solid or
liquid, amount, intensity, and duration).

In the USSR predictive equations
for interception based on forest type and precipitation were
suggested by A. I. Gribov, V. V. Protopopov, V. D. Chernyshev,
and L. P. Kharitonov. In many formulae the main parameters of
forest stands are the socalled leaf area index (i.e. the total
area of all leaves divided by the area occupied by the forest)
and the water-holding capacity of the canopy or a tree. The leaf
area index, according to different authors, ranges in the
temperate zone from 4 to 15 for deciduous forests, from 6 to 18
for pine forests, and from 20 to 40 for spruce and fir stands. As
a rule, the greater the leaf factor the greater is the
interception.

The water-holding capacity of the
canopy or separate tree crowns is the maximum amount of moisture
held by completely wet trees. It depends on the forest type and
the tree age. For deciduous trees it ranges from 0.5 to 1.2 mm;
from 0.9 to 1.5 mm for pine trees; and from 2.8 to 4.6 mm for
spruce and fir, occasionally reaching 6 to 8 mm (Rakhamanov
1981). It should be noted that the water-holding capacity cannot
serve as a direct indication of intercepted amounts since a
considerable portion of precipitation is shaken off by the wind.
These estimates of the water-holding capacity of trees clearly
demonstrate that total interception depends not only on the type
and density of forest but also on storm characteristics: the
greater the proportion of low-intensity precipitation (which is
almost entirely intercepted by all species) the greater is the
total interception.

The most complete and detailed
data on precipitation interception of both snow and rain by
forests were collected at the Valdai branch of the State
Hydrological Institute by Fedorov (1977) and Krestovsky (1969a)
for stands of different species, density, and age. Krestovsky
(1969a, 1969b) and Krestovsky and Sokolova (1980) also analysed
observations made between 1900 and 1980 within the USSR area and
in some countries of Western Europe. They showed that in some
cases data are overestimates, evidently due to storm duration and
intensity not being taken into account and due to reference
precipitation gauges being in small clearings amidst high trees
where precipitation is usually greater than above the forest. For
reliable measurement of interception loss one should observe for
a longer period, perhaps ten to thirty days for liquid
precipitation and for the whole winter period for solid
precipitation, starting from the first snowfall to the start of
intensive spring snow melt when all the snow on the trees
disappears. Precipitation gauges should be installed in clearings
surrounded by rather low, not dense, preferably deciduous,
forests.

Generalized data fulfilling the
above requirements are given in table 1. It shows the portion of
precipitation reaching the soil surface in forest stands of
various type and density regardless of the height and age of the
trees. The large seasonal and annual interception values are
typical of spruce forest, reaching on the average 25% (forest
stand density being 0.8). It should be noted, however, that in
very dense mature spruce forest (with density 0.9 - 1.0) and in
dense young spruce forest, interception amounts to 30-35%,
whereas in dense young pine forest it is 25% of precipitation
during both the warm and the cold periods (Voronkov 1970;
Krestovsky 1969b; Fedorov 1977). Since forest type and density
tend to change with time, the interception of precipitation by
the canopy also changes considerably. Because the interception
value depends on the crown volume, which is related to the weight
of the foliage, interception should relate to the phytomass of
the forest stand; such correlation has recently been established
by the junior author of this review (see fig. 3).

To estimate the hydrological role
of forests one also needs estimates of precipitation interception
by adjacent fields and grassland. Unfortunately, there are few
reliable data on interception by different types of grass and
crops, due to experimental difficulties. Therefore, interception
by grass coenoses is often estimated indirectly, by using the
leaf area index and water-holding capacity. According to
different findings summarized by Rakhmanov (1981), the leaf area
index of grasses and crops ranges from 10 to 85, being, for
instance, 26 for clover, 80 for a , and from 20 to 50 for various grasses.
These values are 1.5 to 2 times greater than those for forest
stands, mentioned earlier. This does not mean, however, that
grasses and crops intercept greater amounts of precipitation:
firstly, the vegetative period of grass and crops is much shorter
than that of deciduous species, to say nothing of the conifers.
Secondly, the rate of evaporation from tree crowns and from
herbaceous vegetation is very different. Detailed experiments on
interception by herbaceous vegetation were carried out by Bulavko
(1971). He used a special design of ground precipitation gauge
and has studied for several years the interception of
precipitation by various crops and grasses in the regions of
Byelorussia. He also thoroughly reviewed the relevant data
available in the USSR. He finds that interception values depend
both on precipitation (type, rate, distribution in time and
amount) and on the vegetative cover (type, density, and
developmental phase). Interception by herbaceous vegetation,
including crops, was observed only during the growing season,
approximately five to six months. During the period of maximum
plant development interception values may reach 30 to 37% of the
monthly precipitation. During this six-month period rye, wheat,
and barley intercept 8 to 9% of precipitation, while potatoes and
perennial grasses intercept 13 to 18%; the respective annual
values appear to be half as much, from 4 to 9% of annual
precipitation. Interception estimates for deciduous forest are
approximately 1.5 to 2 times greater and those for conifers 2 to
3 times greater (see table 1).

Snow Storage and Forest

The forest influence on snow
storage is another aspect of its effect on precipitation. This
aspect has great practical significance, especially for the
plains of the USSR where streamflow is due mainly to the spring
snowmelt. The main method of studying the forest influence on
snow accumulation is by special and standard snow surveys in the
forest and in the open.

Special snow surveys made by
Molchanov (1961) and by Sabo (1956) in the Moscow region showed
that maximum snow storage is in forest clearings. If one assumes
storage here to be 100%, snow storage in pine stands of different
ages and densities ranges from 75 to 93%, in birch and aspen
forest from 84 to 98%, in spruce forest from 67 to 71%, while in
open areas it is between 67 and 82%. Thus in the open snow
accumulation is less than in all types of forest except spruce
forest.

Subbotin (1966), using the data
of the Podmoskovnaya water balance station for a 20year period
and the data of routine snow surveys at 135 meteorological
stations in the northern and central regions of the European
USSR, showed that snow storage in conifer forests is 8 to 28%
greater than in the open, and for deciduous forests the
difference is 25 to 32%, reaching 42% in the south. Much lower
estimates of snow storage in forests were obtained by Rakhmanov
(1962). He used the data of snow surveys in the open and in the
forest conducted at 125 stations of the western, central, and
eastern regions of the forest zone of the European USSR. He
concluded that the amount of snow accumulated in the forest is on
average 17% greater than in the open. He also found that 27% of
the snow surveys showed that snow stored in the forest appeared
to be 5 to 10% less than that in the field. Later Rakhmanov
(1971) studied snow cover formation in the Volga basin (drainage
area: 229,000 km2) using the data of snow surveys at 170
meteorological stations during a 14-year period. It appeared that
in coniferous forests snow storage is 10% greater than that in
the open, while in deciduous and mixed stands the difference
amounts to 27-28%. On average, in the forested area of the
drainage basin snow storage is 20% greater than that in the open
terrain.

There are, however, findings that
do not support the above estimates. For instance, according to
Voronkov (1970), based on 19 years of observations in
experimental areas, snow storage in spruce and spruce-deciduous
forests is usually less (approximately by 9% on the average) than
in the nearby fields; occasionally in some years it may be quite
the reverse. In the Middle and South Urals (Pobedinsky 1979) snow
storage in mature spruce forests is also less than in treeless
areas, whereas snow storage in pine and deciduous forests is
usually the same as in the open.

In the forest zone of Siberia and
the Far East snow surveys generally show that snow accumulation
in forests, especially in mountains, is much greater than in the
open, treeless areas. Protopopov (1975) found that for mixed and
dense conifer mountain forests of Siberia the snow-storage
coefficient is from 1.18 to 1.32; for deciduous and open cedar
stands it is 1.16 to 1.22; in dense cedar and pine forests the
coefficient is 0.80 to 1.10. Detailed studies of snow
accumulation in the forest and in fields in different Siberian
natural vegetation zones were conducted by Lebedev (1982). The
snow storage coefficients in the forest zone were 1.1 to 1.2,
rising to 1.5-1.7 in the steppe-forest zone and to 1.7-2.5 in the
steppe zone. The very large values in the foreststeppe and steppe
zones refer to small forest stands that accumulate additional
snow carried by the wind over vast open areas. Thus wind is an
important factor since it redistributes snow between field and
forest. In the regions of East Siberia, the Far East, and
Sakhalin, forests accumulate substantially higher snow amounts
than fields (Klintsov 1973; Rakhmanov 1981). For instance,
summaries of numerous snow surveys in the Baikal region showed
that snow storage by forests at all elevations is approximately
30% greater than that in fields. This can be attributed primarily
to the big differences in evaporation of snow in the open and in
the forests, especially under conditions of clear skies during
the anticyclones, which are frequent in these regions.

The main results of intensive
surveys of snow storage in forest and open sites carried out
since 1970 for different regions of the forest and forest-steppe
zones of the European USSR, and in the steppes of Kazakhstan, are
presented by A. P. Bochkov (1970); Vershinina (1972, 1979);
Vershinina and Volchenko (1974); Gidromet. (1966); Krestovsky and
Sokolova (1980); Ouryvaev et al. (1965). According to these
studies the long-term ratios of maximum snow storage in the
forest to that in the fields are as follows:

North-west
of the European USSR

1.20-1.30

Central
regions of the forest zone of the European USSR

1.20

North-east
of the European USSR

1.10-1.20

Forest-steppe
zone of the European USSR

1.30-1.80

Steppes of
Kazakhstan

1.30-2.0

In the case of small forest
stands surrounded by vast open space, the ratios are 1.30 to 1.50
for the forest-steppe zone; 1.5 to 2.0 for forest-steppe and
steppe zones of the European USSR; and 2.0 to 3.0 for the steppe
regions of Kazakhstan.

Detailed studies on the maximum
snow storage in forests of different species and density and in
different sizes of open areas have been carried out recently in
the forest zone of the European USSR (Krestovsky and Sokolova
1980; Krestovsky 1980). The results are shown in table 2. The
maximum amounts of snow are accumulated in the clearings in the
middle of the forest as demonstrated by numerous experiments
(Vodogretsky and Krestovsky 1975; Krestovsky 1969a; Fedorov 1977;
Fedorov et al. 1981). In general, snow storage in forests is
about 10-20% greater than that in open terrain, though in dense
spruce forest snow storage is close to that in fields.

TABLE 2. Snow storage in forest and fields by the
start of the spring snowmelt (longterm average relative snow
storage for the forest zone of the European USSR)

Field and forest types

Relative snow storage

Mixed (50%
conifer, 50% deciduous) species

1.00

Deciduous

1.04-1.10

Spruce,
normal density (0.3-0.6)

0.90-0.97

Spruce, high
density (0.7-1.0)

0.80-0.90

Pine

Low mixed
forest, dwarfed species

0.98-1.02

Brushwood
and deciduous low forest

1.04-1.08

Large felled
areas

1.04-1.08

Small felled
areas and clearings surrounded by high forest

1.05-1.15

Mossy forest
(moss bogs)

0.95-1.00

Field
(ploughland, grassland, stubble, slopes of various
orientation and flat areas): average

0.75-0.92

Fields,
northwestern regions

0.75-0.79

Fields,
central regions

0.85

Fields,
northeastern regions

0.87-0.92

The ratios may change markedly
from year to year. During mild winters with thaws they may reach
50% but fall to 5% during very cold frosty winters. The
occurrence of different numbers of mild and frosty winters during
a long-term period largely explains the various values of snow
storage in fields of the north-west, central, and north-east
regions of the European USSR (see table 2). It also explains the
contradictory estimates obtained by different authors for the
same regions. However, even a considerable reduction of snow
storage in the field during mild winters would not necessarily
cause the same reduction of water yield since melt water
saturates the soil and reaches the aquifers.